Download electric field spectroscopy of ultracold polar molecular dimers

ELECTRIC FIELD SPECTROSCOPY OF ULTRACOLD
POLAR MOLECULAR DIMERS
JOHN L. BOHN AND CHRIS TICKNOR
JILA, NIST and Department of Physics, University of Colorado
Boulder, CO 80309, USA
We propose a novel kind of electric field spectroscopy of ultracold, polar molecules.
Scattering cross sections for these molecules will exhibit a quasi-regular series of
resonance peaks as a function of applied electric field. In this contribution, we derive a
simple approximate formula for F(n), the electric field F at which the nth resonance
appears.
The topic of this contribution does not strictly belong at the International
Conference on Laser Spectroscopy, I suppose, since it has nothing whatever to
do with lasers. However, I do intend to discuss a kind of spectroscopy that is
peculiar to polar molecules in an ultracold environment. It turns out that polar
molecules at low temperatures respond really strongly to electric fields,
particularly when the molecules collide. So, by monitoring the scattering cross
section as a function of electric field, you can try to read out information on the
dimer formed by the collision partners. In what I discuss here, I consider the
effect of a dc field; you could think of it as a kind of “dc laser spectroscopy,” if
you like.
Consider a pair of molecules that approach one another in an ultracold
environment, with essentially zero relative kinetic energy. The actual scale of
this energy is assumed to be something like milliKelvin, or perhaps tens of
microKelvin; it depends on whose experiment you’re talking about [1].
Considering that ten microKelvin corresponds to something like 200 MHz of
energy, a cold collision event could in principle probe resonant structure
between the colliding partners on about this scale. This is plenty enough
resolution to distinguish between different ro-vibrational states, for example.
Well, there’s an obvious problem with using these collisions for
spectroscopy. The collision energy, within this temperature-defined width, is
always the same, namely, zero. This seems to imply that you could probe one
very tiny slice of the collisional spectrum really well, but at the expense of
everything else going on in the complex. In conventional spectroscopy, this
would not be a problem, since you could bring in photons of a desired energy
and raise or lower the energy of the collision partners to any other energy level
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2
you want to probe. (In the context of cold collisions, this is in fact done
frequently, and is known as photoassociation [2].)
There is an alternative way to proceed, however. Rather than driving the
molecules to a bound state at a different energy, you could instead shift the
energy of the bound state until it coincides with the energy of the molecules, i.e.,
zero. This is not completely crazy, or at least it’s not without precedent. For
years now experimenters have been using magnetic-field Feshbach resonances in
alkali atoms to make resonant states coincide with the threshold of the scattering
continuum. This has been a very fruitful tool for engineering the mean-field
interaction energy of ultracold gases, but it is something else, too. For a realistic
two-body interaction potential to reproduce such a resonance at the correct field
is quite a severe constraint on the potential [3]. The point is, field variation of
cold collision cross sections can really be a useful probe of the interaction, i.e, a
spectroscopic tool.
-3
10
-5
σ (cm2)
10
-7
10
-9
10
10
-11
10
-13
0
1000
2000
3000
Electric Field (V/cm)
4000
5000
Figure 1. Elastic scattering of SrO molecules at a collision energy of 10-12K, as a function of electric
field. This scattering produces a surprisingly regular series of resonances.
The effect of an electric field on polar molecules is likely to be much more
profound, since electric forces are generically stronger than magnetic forces in
molecules. And sure enough: in Figure 1 I show the electric-field-dependent
elastic scattering cross section of ultracold SrO molecules. These molecules are
assumed to be in their electronic (1Σ), vibrational (v=0), and rotational (J=0)
ground states, and to collide at an energy of 10-12 K. This figure is reproduced
from our recent study of electric field resonances [4], and the details of the
scattering model are explained there.
The main point of figure 1 is that the cross section is dominated by a
strikingly regular series of resonances. In Ref. [4] we explain that these
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resonances originate in the purely long-range dipole-dipole interaction between
the molecules. Roughly, the field can change the degree of polarization of the
molecules, hence their dipole-dipole interaction. Ref. [4] also shows that the
exact position and spacing of the resonances carries information about the
“short-range” physics, where the molecules actually collide and probe their
potential energy surface. (Also appearing in this figure, although far less
prominent, are various Feshbach resonances that describe excitation to higherlying rotational and fine-structure states. Obviously they will also carry
information about the interaction that excited these degrees of freedom.)
This discussion should sound vaguely familiar. The backbone of atomic
spectroscopy is the Rydberg series. Any atom with a singly-excited electron
has a spectrum of energy levels that scales as -1/n2. Well, no, of course that’s
not true. In fact, the spectrum of such an atom scales as -1/(n-µ)2, where µ is a
quantum defect [5]. The role of the quantum defect, as all good spectroscopists
know, is to encode the electron’s interaction with all the other electrons in the
parent ion. The entire series of energy levels is described by a simple formula
that is flexible enough to apply to any atom.
What I would like to do here is to derive an analogue of the Rydberg
formula for an electric-field spectrum like the one in Figure 1. In other words, I
would like an analytic expression F(n) that gives the electric field value F at
which the nth resonance is observed. This will not be the world’s greatest
derivation, but it will get the basic physical ideas right.
Let’s first note the following. In spectra such as that in Fig.1 that I’m
considering, the collision energy is nearly zero, and it is the electric field that is
being scanned. The resonant state therefore extends to infinite intermolecular
separation R. In this case, the relevant interaction potential is described by the
largest-R part of the adiabatic potential energy curve. Well, we’ve discussed the
form of this curve previously [6]. The lowest potential, the one that caries the
resonances, is mostly s-wave in character (i.e., it has partial wave angular
momentum l=0). But the dipole-dipole interaction vanishes by symmetry for swaves, so the only effect of the dipolar interaction is to mix s-waves with nearby
d-waves (l=2).
It turns out that this means two things. First, in zero field the effective
interaction scales as –C6/R6. Second, in nonzero field, the interaction quickly
turns over to a –C4/R4 behavior, where the effective C4 coefficient is given by [6]
2
 η 2  µ4 m
r .

(1)
=
C4 
 1+η 2  h2


Here η = 2µF / ∆ represents the electric field F in units of the critical field
F 0 = ∆ / 2 µ that separates the quadratic and linear Stark effect regimes; ∆ is the
energy splitting between the given rotational level and the next higher one; and
mr is the reduced mass of the collision partners. Equation (1) includes only the
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perturbation due to the l=2 partial wave, but you can easily extend it to higher
partial waves, if you like. Notice that the value of C4 saturates as you go to the
high-field limit F>>F0. Well, of course: once the molecules are polarized, the
field can’t do much more to change the situation. (I do not consider here fields
large enough to distort the electronic wave functions, which would be another
story.)
Since I’m talking here about field-dependent resonances, I will concentrate
on the field-dependent 1/R4 part of the interaction. At zero collision energy, the
WKB phase for this potential is easily calculated. To identify a bound state, you
set this phase equal to an integral multiple of π:
φWKB = ∫a∞ dR
2 mr C 4
=
2
4
h R
2 µ 2 mr  η 2 
.


2
 1+η 2  = nπ
ah


(2)
Here I have imposed an arbitrary small-R cutoff radius, a. And it’s a good thing,
too, since the integral would diverge if I were to let R → 0 . But anyway, the
potential is clearly not proportional to 1/R4 all the way to small R, and besides,
whatever goes on for R<a is going to be accounted for in the quantum defect.
Now you could just invert (2) to find the resonant field at which the nth
bound state appears. As a fitting formula, this leaves something to be desired,
however, since the result would depend explicitly on the unphysical cutoff radius
a. A better use of the quantization condition would be to exploit the saturation
behavior, and to write
n
η2
,
=
2
~
n∞
1+η
(3)
where n~ ∞ = 2 µ 2 mr / πah 2 is the approximate total number of bound states in
the potential in the limit of large electric field. In other words, when the field is
~∞ . Having made this correction, then you could then
large,η → ∞ and n → n
invert (3) to find the set of resonant field values F(n).
Oh no, wait, I’m sorry, there is one more thing. In practice, it’s more useful
to append the quantization condition (3) to read
η2
n + n0 .
=
2
n~∞
1+η
(4)
In this expression, n0 / n∞ represents the additional part of the phase shift arising
from the short-range part of the potential. Here is our analogue of the quantum
defect. Finally, when you do actually invert the expression (4) to find the
resonant fields, you get
F ( n) = F ′
n0 + n .
n∞ − n
(5)
5
where n∞ = n~∞ − n0 , and F’ is expected to be on the order of F0.
Does this work? Surprisingly, yes. Look at Figure 2, where I have plotted
the resonant fields F(n) versus n for both the SrO example in Fig.1, and for
RbCs. In both cases I have fit to the formula (5), and shown the results as solid
lines. The fit is not half bad, and clearly gets the general form of the spectrum
correct.
5
F (kV/cm)
4
3
2
SrO
RbCs
1
0
0
5
10 15 20 25 30 35
n
Figure 2.Electric field values F(n) at which the nth resonance occurs, as seen in close-coupled
calculations of SrO (squares) and RbCs (triangles) cold collisions. Solid lines represent fits to the
formula in Eqn. (5).
A few remarks are in order. First, the actual value of n is pretty arbitrary,
since I have no idea how many bound states there really are. In fact, I have
anchored the calculation in the final number of bound states after the field has
saturated the interaction. Therefore, it actually makes more sense to count the
resonances “backwards,” from n∞ down. (Note that n∞ need not be an integer,
of course.) This is actually a typical way to count the very most weakly-bound
states of potentials [7].
Second, the assumption that the long-range potential scales with R as 1/R4 is
not true at zero field. Thus, in order to get a suitable fit for the RbCs spectrum, I
had to ignore the first four calculated resonances. Third, by no means is (5)
intended to be a rigorous result. Rather, it is a simple and comfortable way to
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characterize the series in a simple formula. It should, for example, provide an
estimate of how many resonances to expect for a given molecule in a given field
range. Plus, it strongly suggests that there should exist a simple formula.
Someday somebody should go back and find the correct formula in a more
rigorous way.
Acknowledgments
This work was supported by the National Science Foundation, and by a grant
from the W. M. Keck Foundation.
References
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31, 149 (2004).
[2] J. Weiner, V. S. Bagnato, S. Zilio, and P. S. Julienne, Rev. Mod. Phys. 71, 1
(1999).
[3] N. R. Claussen, S. J. J. M. F. Kokkelmans, S. T. Thompson, E. A. Donley,
and C. E. Wieman, Phys. Rev. A 67, 060701 (2003).
[4] C. Ticknor and J. L. Bohn, physics/0506104, to appear in Phys. Rev. A.
[5] H. Friedrich, Theoretical Atomic Physics (Berlin: Springer,1998).
[6] A. V. Avdeenkov and J. L. Bohn, Phys. Rev. A 66, 052718 (2002).
[7] R. J. LeRoy and R. B. Bernstein, J. Chem. Phys. 52, 3689 (1970).